Sepharose linked anti-immunoglobulin induced a 3-5 fold greater incorporation of thymidine into DNA of B cells than the soluble anti-immunoglobulin from which it was derived. The greater incorporation was accounted for on the basis of the presence of at least two subpopulations only one of which can be induced to proliferate by soluble anti-immunoglobulins whereas both populations were activated by Sepharose linked antibody. The two populations were distinguished by criteria as follows: 1) a difference in the rate and extent of cell inactivation resulting from adverse conditions; 2) a difference in distribution on Percoll gradient centrifugation; 3) a difference in the number of cells induced to proliferation by soluble and insoluble anti-immunoglobulin as judged autoradiographically; 4) a difference in the liability of the two subpopulations to treatment with pronase and 5) it was observed that soluble anti-immunoglobulins delivered an inhibitory signal to only those cells selectively inducible with anti-immunoglobulin covalently linked to Sepharose. The inhibitory signal that soluble anti-immunoglobulin delivered to cells activatable by insoluble anti-immunoglobulin was demonstrated to be due to the antibody combining site on anti-immunoglobulin. Although the inhibition results at least initially from the interaction of soluble anti-immunoglobulin with surface immunoglobulin, the inhibitory signal was irreversible in that cells allowed to recover their receptors, subsequent to their internalization induced by the soluble antibody, still do not respond to insoluble antibody. Chloroquine and monensin, known to inhibit internalization related events yielded inhibition of proliferation that paralleled the inhibition by a specific competitive ligand, rabbit immunoglobulin, and also inhibited receptor reappearance. While soluble anti-immunoglobulins induced the internalization of surface receptors, insoluble antibody was without effect. At concentrations of anti-immunoglobulin sufficient for maximum activation Sepharose beads bearing a relatively low density of ligand per bead were incapable of inducing the proliferation observed with beads bearing a relatively high density per bead.